Safe and sane multi-item storage removal (#11490)

* Fix overlay prefix removal result

* Second part of the overlay prefix removal fix.

* Report only items deleted from storage in clear_prefix

* Fix kill_prefix

* Formatting

* Remove unused code

* Fixes

* Fixes

* Introduce clear_prefix host function v3

* Formatting

* Use v2 for now

* Fixes

* Formatting

* Docs

* Child prefix removal should also hide v3 for now

* Fixes

* Fixes

* Formatting

* Fixes

* apply_to_keys_whle takes start_at

* apply_to_keys_whle takes start_at

* apply_to_keys_whle takes start_at

* Cursor API; force limits

* Use unsafe deprecated functions

* Formatting

* Fixes

* Grumbles

* Fixes

* Docs

* Some nitpicks 🙈

* Update primitives/externalities/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Formatting

* Fixes

* cargo fmt

* Fixes

* Update primitives/io/src/lib.rs

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

* Formatting

* Fixes

Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

substrate/frame/staking/src/testing_utils.rs

@@ -36,9 +36,11 @@ const SEED: u32 = 0;
 
 /// This function removes all validators and nominators from storage.
 pub fn clear_validators_and_nominators<T: Config>() {
+	#[allow(deprecated)]
 	Validators::<T>::remove_all();
 
 	// whenever we touch nominators counter we should update `T::VoterList` as well.
+	#[allow(deprecated)]
 	Nominators::<T>::remove_all();
 
 	// NOTE: safe to call outside block production